Want to build a block theme from scratch? You only need three essential files to create a valid block theme.
First is style.css – even if it’s empty, it must contain the theme header information that WordPress uses to identify your theme. Modern block themes often have no actual styles in this file.
Second is theme.json with at least a version number specified. This file controls all your theme’s settings and styles, even if you start with just the basic version declaration.
Third is templates/index.html – your fallback template file. Unlike classic themes, this can be completely empty when starting a new block theme.
That’s it! Just three files and you have a functional block theme foundation ready for development through the site editor.